1. Field of the Invention
The present invention relates to a recording apparatus which discharges ink by time-division driving of a recording head arraying a plurality of nozzles and a method of controlling the recording apparatus.
2. Description of the Related Art
In recent years, with the widespread use of information-processing equipment, a recording apparatus as its peripheral equipment has also rapidly come into wide use. Among these recording apparatuses, since an ink jet recording apparatus has various advantages, it is adopted in many recording apparatuses. The ink jet recording apparatus scans a recording medium with a recording head, discharges ink droplets from the recording head in the scanning, and executes recording. The advantage of the ink jet recording apparatus is easy miniaturization, and color recording can be relatively simply performed.
Among the ink jet recording apparatuses, by a thermal ink jet method, high integration of a discharge mechanism is relatively easily performed and discharge ports for discharging ink can be arrayed at a high density. The thermal ink jet method utilizes bubbles generated by thermal energy to discharge ink. Owing to high density of the discharge ports, the recording apparatuses can be miniaturized and further, a high-quality image can be recorded at a high speed. In the recording apparatus using a recording head that arrays such many discharge ports, in order to simultaneously drive the entire array of discharge ports to discharge ink at the same timing, a large-capacity power source will be required. Thus, a time-division driving method has been adopted. The time-division driving method sequentially drives the predetermined number of discharge ports which are arranged on the recording head within a period of a driving cycle. More specifically, the time-division driving method typically divides the entire array of discharge ports of the recording head into a number of groups and bit by bit changes timing of driving for each group. Since the number of discharge ports to be simultaneously driven is reduced by executing this time-division driving, the capacity of a power source required for the recording apparatus can be reduced.
On the other hand, the ink jet recording method handles ink which is a fluid. This may cause various inconveniences due to a hydrodynamic phenomenon. For example, when ink is discharged from a certain discharge port, a pressure change generated at that time is propagated to adjacent discharge ports through an ink flow path to vibrate an ink interface of the discharge ports. This causes a significantly unstable state. Due to the vibration of the ink interface of the discharge ports, there has also been cases in which discharge ports after discharge of ink is not sufficiently filled with ink (unstableness of ink film). If ink is discharged in such an unstable state, a position to impact ink droplets on a recording medium may be shifted and the amount of ink droplets to be discharged from the discharge ports may fluctuate. The shift of the position of ink droplets or the fluctuation of the discharge amount of ink can result in an uneven density and a white streak on an image which is recorded on a recording medium. Since the color of a recording face of the recording medium is white and a white line is generated on the image, it is referred to as the white streak.
In order to solve a discharge failure due to the vibration of the ink interface, the level of a negative pressure generated in a liquid chamber can be approximated to a normal pressure by optimizing the timing and the discharge amount of time-division driving during discharge of ink (Japanese Patent Application Laid-Open No. 05-084911). Japanese Patent Application Laid-Open No. 05-084911 describes a technique of approximating the negative pressure generated in the liquid chamber to a normal pressure by optimum time-division driving, in which ink is discharged with a small and stable amplitude of the vibration in ink-refill, and a driving frequency is enhanced.
However, even when the time-division driving is executed so as to reduce the amplitude of the vibration in the ink-refill, there has been the case in which an impact position of an ink droplet adhering to a recording medium is shifted. In order to achieve recording with a high image quality which is required in a recent recording apparatus, there are recording heads that include a nozzle array of discharge ports arranged at a high density or an increased number of nozzle arrays. When ink is continuously and sequentially discharged from the discharge ports arranged at a high density, an air current is generated between the nozzle face of the recording head and the recording medium by discharged ink droplets. The generation of this air current places the vicinity thereof in a state of a negative pressure. Thus, a flying direction of the ink droplet discharged from nozzles can be changed. This deviates the flying direction of the ink from a desired flying direction. As a result, the impact position (dot position) of the recording medium can be shifted. As described above, due to a white streak thus generated, an image quality is degraded.
The higher a density (Duty) of an image recorded by a single recording scan, a white streak generated on a recording medium may be more noticeable. This is because when a high-Duty image is recorded, a generated air current becomes larger, so that the amount of shifts in a flight direction of an ink droplet is increased.